Typical example of methods for preparing samples for SEM include freeze fracturing, microtomy, and mechanical polishing, which are suitable for samples having relatively large sizes and being relatively strong. With the freeze fracturing, however, it is difficult to obtain a target observation surface, and it is also difficult to obtain a smooth section. The microtomy is a technique of producing a sample section with a knife, with which it is difficult to obtain a smooth section of a relatively soft sample or the like owing to the stress, and which thus requires proficient skills of operators.
With the mechanical polishing, since a sample section is typically embedded with resin and polished with water or a lubricant, it may be difficult to perform the resin embedding and polishing on a relatively small and soft sample or the like, and it is difficult to perform the mechanical polishing on a sample that should avoid water or the like.
Preparation of a sample section using an ion beam has come into common use recently as a method capable of preparing a sample section with no stress. Ion milling apparatus and method are proposed as means for preparing a sample section by irradiating a sample with an ion beam, in which a shield using a material with a small sputtering yield is disposed on a top surface of the sample in an evacuated sample chamber, a part of about 50 to 200 μm of the sample is exposed from an end surface of the shield and irradiated with an ion beam from the top surface side (on the side of the shield) of the sample, a physical sputtering phenomenon is used to sputter atoms from the sample surface, and a milled surface in a shape along the end surface of the shield is obtained with no stress.
Typically, ion milling conditions of a sample for an electron scanning microscope that are often used are an accelerating voltage of about 10 kV or lower and an ion beam current of about 200 μA or lower. In this case, the quantity of heat applied to the sample by the ion beam irradiation will be about 2 J/s or lower, but since the range of the ion beam irradiation may be a half width of the sample milled surface of about 300 μm and a processing time exceeding several hours, the temperature rise is not negligible in applying ion milling to a sample with a low melting point such as a polymer material.
In order to curb rise in temperature, PTL 1 explains that a braided wire for cooling the shield is connected to the shield. In addition, PTL 2 discloses a section sample preparation method in which a board for radiation is disposed between the shield and the sample so that heat generated by ion beam irradiation is radiated to the sample holder.